Semiconductor elements such as transistors and diodes are used in circuits such as switching power supply circuits and inverter circuits. These semiconductor elements are required to have high breakdown voltage and low on-resistance. Then, there is a trade-off relation determined by an semiconductor material as a relation between a breakdown voltage and an on-resistance.
Along with progress in technology development, the low on-resistance has been realized in semiconductor elements up to near the limit of silicon which is the major semiconductor material. It is necessary to change the semiconductor material in order to further improve the breakdown voltage and further reduce the on-resistance. It is possible to improve the trade-off relation determined by the semiconductor material and to realize remarkable high breakdown voltage and low on-resistance by using a nitride semiconductor, such as gallium nitride and aluminum gallium nitride, as the semiconductor material of the semiconductor element.
In a transistor using the nitride semiconductor, however, charges are injected into an insulating layer formed on the nitride semiconductor, which results in degradation of characteristics such as a fluctuation of a threshold voltage, an increase of a leakage current of the insulating layer, and an increase of an on-resistance.